Which is made possible in variable conditions (paceline position, grade, wind, road condition, etc) by being able to change gear ratio. For a given power, there's a given cadence where you'll see peak efficiency. Spin out and you're wasting energy turning your feet around, push high force onto slow cranks and work=force*distance wastes the efforts of the human leg. What might be a perfect gear one moment can be a terrible gear the next.

Training can mitigate the disadvantages of single-speed. Spinning a lot can, well, smooth your spinning; and there's definitely technique to keeping up good form at ultra-low cadence, both in and out of the saddle. But this doesn't solve the basic problem.